As is well known, a wind turbine is a machine that converts the kinetic energy in wind into mechanical energy. If the mechanical energy is used directly by machinery, such as a pump or grinding stones, the machine is usually called a windmill. If the mechanical energy is converted to electricity, the machine is called a wind generator. Wikipedia, http://en.wkipedia.org/wiki/Wind_turbine. Wind turbines can be further categorized by structure and orientation based on the axis about which the turbine rotates. Turbines that rotate about a horizontal axis are called horizontal-axis wind turbines (HAWT), whereas those that rotate about a vertical axis are called vertical-axis wind turbines (VAWT). HAWTs are more common than VAWTs. Wikipedia, supra.
In principle, producing electric power with wind is a simple process. Most HAWT turbines have three large blades mounted to a rotating hub. The blades are aerodynamically designed to turn as easily as possible when the wind blows on them (the number of blades may vary). The turning blades spin a shaft, which connects through a gearbox to a generator that produces electricity. The gearbox and generator are mounted in a nacelle which, in turn, is mounted atop a tower. As the wind blows over the turbine blades they create “lift”, much like an airplane wing, and begin to turn. The spinning blades turn a low-speed shaft at a relatively low speed, usually 30-60 rpm. The gearbox connects the low-speed shaft with a high-speed shaft that drives the generator. The gearing also boosts the rotation speed of the high-speed shaft to the operating speed of the generator. This operating speed may vary, but is usually in the range of 900-1800 rpm. This rapidly spinning shaft drives the generator to produce electric power. The generator's electrical output is connected to the larger electrical grid. Typically, large capacity generators provide polyphase voltages at a controlled frequency synchronized to the grid. The generator outputs are connected to the grid via suitable transformers.
The blades themselves can also be turned, or pitched, about their longitudinal axes, out of the wind, to control the rotor speed and keep the rotor from turning in winds that are too high or too low to produce electricity. They can also be pitched to a “feather” position to prevent rotation in the event of an emergency. (The wind turbine also typically includes an emergency braking system to stop rotation in the event of an emergency.) The blades are rotated about their longitudinal axes by a pitch control system. There are several different ways of doing this, including actuators and motors. The pitch control system, which comprises motors or actuators and associated power supplies and control electronics, is conventionally mounted in the rotating hub of the turbine. Power is supplied to the pitch control system from slip rings which transmit power from a stationary bus/supply mounted in the nacelle. The power supply for the pitch control system can come from a number of sources. It can be provided by the main grid itself via appropriate transformers, or it can be provided by the generator driven by the turbine.
Historically, wind turbines have contributed a very low percentage of the world's energy demands. But depletion of natural resources such as oil and natural gas, associated higher prices for these resources, and political ramifications associated with reliance on foreign oil, are changing the energy generation landscape. The industry is responding with turbines of higher capacities (ratings of 1.5 MW or more), better technology, and wind farms having large numbers of wind turbines. As recently reported by CNNMoney.com, “Wind energy industry sources reported that approximately 15,000 megawatts of new wind energy generation capacity was installed worldwide in 2006, an increase of 25 percent from 2005. The industry has maintained an average growth rate of more than 17% for the past five years, and industry estimates project a similar growth rate and a total wind energy equipment market value of more than $180 billion for the next five years.” http://money.cnn.com/news/newsfeeds/articles/prnewswire/LAM00302072007-1.htm. These statistics and forecasts are confirmed by E.ON Netz, the German transmission system operator of the E.ON Group, who reported in 2005, “In 2004, Germany was once again the global world leader in the production of wind power. At the end of 2004, wind energy plants with an installed capacity of 16,400 MW supplied the German electricity grids . . . . According to grid studies by the Deutsche Energie-Agentur (dena), wind power capacity in Germany is expected to increase to 48,000 MW by 2020, around a threefold increase since 2004 . . . . This means that Germany remains the world's undisputed number one generator of wind energy. In 2004, Germany accounted for approximately one third of the world's and half of Europe's wind power capacities . . . . In total, German wind farms generated 26 billion kWh of electricity, which is around 4.7% of Germany's gross demand.” Wind Report 2005, E.ON Netz. In the past, when wind turbines played a negligible role in power generation, they could be largely ignored when considering grid stability. This is no longer the case.
In response to this growth in the wind turbine industry and its impact on the national grid, the Federal Energy Regulatory Commission (“FERC”) has proposed minimum requirements for wind plant response to certain low-voltage conditions on the utility power grid. These requirements require that wind turbines stay connected to the grid during prescribed transient “grid-loss” conditions. Similar requirements are being mandated by grid connection and regulatory authorities throughout the world. Generally, they describe the voltage falling immediately at t=0 to a substantially reduced level such as 10 or 15% of nominal line level and then gradually returning to at least 80% of nominal line level within three seconds of t=0. The levels are considered to be all three phases combined and not with regard to the individual phases. The aggregated requirements of FERC, E.ON Netz (Germany), HECO (Hawaii), and the Spanish grid authority, for example, can all be satisfied by one simplified power loss profile described as follows: the pitch control system should continue to operate normally when the AC mains voltage level falls below 80%, and as low as zero, and remains below 80% for at least as long as three seconds, at which time the AC main level returns to a minimum of 80% of nominal line level.
This continued operation of the pitch control system is referred to in the industry as “ride-through” capability. It broadly describes the ability of the pitch control system to function during a “grid loss” condition, i.e., a condition which cuts power to the pitch control system for any number of reasons. Interestingly, not everyone in the industry defines “grid loss” in the same way, or attempts to solve the same problem, much less in the same way. For purposes of this patent, we define grid loss as any condition that interrupts power to the pitch control system of a wind turbine/generator. This can be caused in a number of ways, including but not limited to, a fault in the main grid; a problem with the pitch control AC power supply (short or other fault); a defective slip ring; a broken conductor, or the like. To understand the present invention, it is important to note that the pitch control system is traditionally housed within the rotating hub of the turbine. The system needs power to operate. As is well-known in the electrical arts, the most common way of transmitting power from a stationary source to a rotating load is via slip rings. It should also be appreciated that “grid loss” as defined herein can occur on either side of the slip rings—on either the stationary or rotating side of the circuit. It is important and necessary to detect the loss wherever it may occur, and take corrective action accordingly. With this in mind, we briefly review patented inventions and published patent applications by others who have addressed problems with wind turbines.
U.S. Pat. No. 6,921,985 (Janssen et al.) discloses a low voltage ride-through solution for wind turbine generators. The patented invention includes a turbine controller and blade pitch control system which are connected to a first power source (AC grid) during a first mode of operation, and to a second source (backup power) during a second mode of operation, i.e., during grid power loss. The turbine controller senses a transition between the two power modes and varies the pitch of one or more blades in response to the transition. The patent also teaches that the turbine controller detects a low voltage event through coupling to sensors which provide data indicating the status of various wind turbine generator system components, for example, rotor speed and generator output voltage. When low voltage is sensed the controller transitions between AC power and UPS power. Janssen et al. measure grid voltage at the transformer, i.e., on the stationary side of the pitch control circuit. Unfortunately, what this means is that if the invention of Janssen et al. was to lose a slip-ring, the patented invention wouldn't detect it.
United States Patent Application Publication No. 2005/0122083 (Erdman et al.) discloses a generator with utility ride-through capability. This publication teaches measuring voltage from either a single phase or from all three phases of the low side of the main grid transformer, but teaches that amplitude of the signal is unimportant. The application teaches that frequency and phase are much more important. The system uses a phase-locked loop scheme to produce a current command signal in a scheme which controls frequency and phase of the generated voltage from the wind turbine, and maintains the phase-locked loop signal during a brief fault. Erdmann et al. are silent as to the exact voltage measurement point, saying only that, “A frequency and phase angle sensor 8 is connected to the utility grid at an appropriate point to operate during a fault on the grid.” (Paragraph 31). It appears that the reference does not teach measuring at the slip rings on the rotating side of the pitch control circuit. Also, Erdmann et al. is largely silent as to powering the pitch control system during a ride through, i.e., the publication doesn't teach a pitch control system arranged to operate during grid loss.
United States Patent Application Publication No. 2006/0267560 (Rajda et al.) discloses a device, system, and method for providing a low-voltage fault ride-through for a wind generator park, i.e., for a plurality of wind turbine/generators. The system uses a resistor bank to absorb power and a control system that maintains collector bus voltage above a threshold voltage during the duration of low-voltage condition on the power grid. The invention in this application monitors voltage levels on the collector bus, i.e., the bus coupled through a transformer to the wind turbine driven generator, and not on the rotating side (slip ring side) of the pitch control circuit.
United States Patent Application Publication No. 2007/0057516 (Meyer et al.) discloses a pitch control battery backup method and system. The published application describes an invention which uses a passive method for controlling a pitch control system via a charged backup battery which provides no power to a DC link when full AC power is available, but uses power from the DC link (including a capacitor) when AC power is lost or dips below a threshold level. The patent application is silent as to the method used to sense AC power loss, mentioning “sensor” only generically.
What is needed, then, is a method and apparatus for grid loss ride-through for a wind turbine pitch control, and especially for a method and apparatus that senses grid loss on the rotating side of the pitch control circuit, i.e., proximate the slip rings.